1. Field of the Invention
The present invention relates to the thermal treatment of calcareous mineral raw materials and/or mineral raw materials containing other carbonate constituents, particularly for the manufacture of calcined products such as burnt lime, cement, magnesite, dolomite and the like. The raw materials are preheated in zone fashion, first in a preheating zone in direct heat exchange with suspending hot gases from a calcining section, are calcined in the calcining section, and then sintered as required and finally cooled in a cooling zone.
2. Description of the Prior Art
It is standard procedure to employ feed material in the form of raw meal when calcining mineral-type raw materials of the type previously mentioned containing carbonate constituents in a calcining aggregate, particularly one having a suspension-type preheater.
One of the main reasons for this approach is that the heating time of the mineral particles in the gas stream significantly depends on the particle size as shown in the diagram and explained in the text in the publication "Zement-Data-Book" by Duda, Bauverlag GmbH, Wiesbaden and Berlin, 2nd Edition, 1977, page 381, FIG. 20.3.
To take an example, the transit time of a raw material particle with a fineness between 0.1 and 0.2 mm through a four-stage cyclone heat exchanger having a height of approximately 50 meters amounts to about 50 seconds from the time the raw material is admitted up until it reaches the rotary kiln intake. Raw meal is heated from about 50.degree. C. to about 800.degree. C. during this transit time, and the ascending kiln gases are cooled from about 1100.degree. C. to about 330.degree. C. The gas and the material rate in the ascending conduits amounts to about 20 to 22 meters per second. Due to this good and spontaneous heat transmission effect, previous efforts have been directed at delivering the material to be calcined to the system at a meal fineness at which about 100% is less than 200 microns.
It is known from the same aforementioned publication according to Chapter 5.1 entitled "Mahlung in der Zementerzeugung" that about 85% of the energy to be expended for the calcining material is involved in the grinding and comminution of the raw material, and that 75% of that amount is involved in the grinding of the raw material and of the clinker.